The present invention relates to the electrical, electronic and computer arts, and more specifically, to analog, mixed signal design and communications technologies.
Modern electrical data transmission systems in networking and computing systems typically employ active SERDES (serializer/deserializer) based line drivers and receivers connected by an external passive differential transmission line, as shown in FIG. 1. Note the transmitter 101, receiver 103, and differential transmission line 105. The differential transmission line has well-known significant advantages for high data rate transmission, including improved line crosstalk, common mode noise suppression, and reduced EMI radiation, as compared to single-ended data links. The transmitter 101 includes serializer 111 with data input, coupled to differential line driver 113. The receiver 103 includes differential line receiver 115 coupled to deserializer 117 with data output.
As data rates increase from 10 Gbs/s to 28 Gb/s to 56 Gb/s to 100 Gb/s and higher in modern systems, a significant problem arises in the passive differential line due to a potential time delay difference between the two distinct lines in the differential system. This time delay difference arises due to practical non-idealities in constructing the transmission line, such as differing impact of fiber weave in a board laminate on one physical line vs. a second line, or small variations in line width or height with respect to the ground reference plane, which impact the characteristic impedance or “ZO” of the line and can impart different delay characteristics.
The delay difference is referred to herein as “PN skew,” where P line 107P represents one of the differential pair lines 105 (positive signal polarity reference) and N line 107N represents the second of the differential pair lines 105 (negative signal polarity reference) as diagrammed in FIG. 1. The PN skew may be improved in manufacturing by using higher-cost techniques, but in typical modern systems can be in a range, for example, of 0.5 ps/inch to 1 ps/inch delay difference between the P and N lines. For a typical 30-inch long microstrip transmission line channel, this can result in 15 ps to 30 ps delay difference between the P and N lines of a differential pair. For a modern high data rate system such as a 100 Gb/s system, the transmission symbol rate of the system may be well over 50 Gbaud/s, resulting in a symbol period of ˜20 ps. When a differential line has a delay difference of 1 symbol period, it will exhibit essentially infinite transmission loss at the BAUD/2 frequency, greatly impairing the ability of a transceiver to send data through the line. The skew problem can be mitigated at the board design level by either using shorter channels so less PN skew is built up, or resorting to expensive manufacturing techniques which inherently reduce the skew. Using shorter channels may be impractical in many system designs, and increased expense of building the passive channel can make the entire system unprofitable to construct.